All relative pronouns except 'that' can be used in non-defining clauses. So here the correct option would be "which".

I looked up the definition of non-defining relative clauses (link below) and it matches your definition https://learnenglish.britishcouncil.org/en/grammar-reference/relative-clauses-non-defining-relative-clauses However, the CAE possible answers, as you point out, are not consistent with this definition. When I tried the exercise, I agreed with CAE that all three answers were possible and natural. I personally would have omitted the relative pronoun. I think the definition is more purist than reality. There are some cases when it would not feel right to use "that" or to omit the relative pronoun, as in the example on the British Council page linked). The difference could relate to the structure of the phrase in the relative clause. The new product launch, _____ we had all worked so hard for, was a disappointment. - Here the relative clause has its own subject -"we" and the "launch" is the object of the relative clause. For some reason, we are now more relaxed about the choice of relative pronoun. The film, which stars Tom Carter, was released on Friday. - Here the relative clause has no subject of its own and the "film" remains the subject of it. So perhaps we insist on "which" to emphasise the link with the subject. I am not a grammar expert so please treat with some caution. But my theory seems to make sense. If you want, you could give me a few example sentences of your own and we could test the theory.

All my books say the same - in a non-defining clause you can't replace which by that and you can't omit it. So the only option here is 'which'. Another thing to tell you that it's a non-defining clause is that you have it in commas. There is no comma before a defining relative clause.

I would simply put it down to a misprint. It happens.

@Kev Dean Thank you very much. But i am still in doubt. Since you say it's a non-defining clause, why "that" can be filled in? The textbook said "that" can not be used in a non-defining relative clauses.